Dual panel liquid crystal display device

ABSTRACT

A LCD device includes first and second LCD panels stacked one on another. Each of the first and second LCD panels includes a pair of transparent substrates, a liquid crystal layer sandwiched therebetween, and a pair of polarizing films sandwiching therebetween the pair of transparent substrates. A light diffusion layer having light diffusion function is interposed between the first LCD panel and the second LCD panel. The light diffusion layer reduces the intensity of the light passed by the first LCD panel, thereby alleviating the periodicity of the arrangement of dark areas and bright areas to alleviate the moire caused by light interference.

This application is a divisional of U.S. application Ser. No.11/736,513, filed Apr. 17, 2007, now U.S. Pat. No. 7,916,223, issuedMar. 29, 2011.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2006-114085 filed on Apr. 18, 2006, andJapanese patent application No. 2007-108283 filed on Apr. 17, 2007, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) device,and more particularly to a direct-view-type LCD device capable ofachieving a higher contrast ratio.

2. Description of the Related Art

LCD devices have the advantage of realizing a higher definition with alower power dissipation and are used for a wide range of applicationsfrom a small-screen cellular phone to a large-screen television monitor.However, there is a defect in the LCD device that the contrast ratio ofthe LCD panel alone in a dark environment is lower than that of a CRT,that (3000:1) of a plasma display panel, which are also used as atelevision monitor similarly to the LCD panel, and that of afield-emission display panel called FED/SED, and is at most on the orderof 1000:1. Therefore, there is pointed out the problem of insufficientfeeling of live performance during representing image sources such as amotion picture having a higher power of expression especially in thedark area.

In order to solve the above problem, there has been developed atechnology for controlling the light intensity of the backlightaccording to the image to be displayed, thereby improving the contrastratio on the display screen, with the contrast ratio of the LCD panelitself being left intact. In a conventional backlight unit having asurface-emission light source, however, a cold-cathode tube having anarrow dynamic range is used as the light source. Thus, improvement ofthe contrast ratio by controlling the light intensity of the backlightaccording to the image to be displayed is limited to around 2000 to3000:1.

It is to be noted that the cold-cathode tube of the backlight unit has ashape of rod. Thus, if there are a high luminance area and a lowluminance area concurrently represented on the same screen of the LCDdevice, the luminance of the backlight cannot be regulated area by area,resulting in a poor improvement of the contrast ratio obtained by theluminance control of the backlight. Therefore, if the image representedon the screen has a higher luminance area and yet is desired toemphasize the reproducibility in the lower luminance area, the effectivecontrast ratio is lowered due to the presence of the higher luminancearea.

In order to solve the above problems, the contrast ratio of the LCDpanel should be drastically improved. However, as described before, thecontrast ratio of the LCD panel alone is at most about 1000:1.Techniques for manufacturing LCD devices capable of remarkably improvingthe contrast ratio thereof without improving the contrast ratio of theLCD panel itself are described, for example, in Patent PublicationsJP-1989-10223A and JUM-1984-189625A. In these technologies, amulti-panel LCD structure wherein two or more LCD panels are stacked oneon another is employed in a LCD device to reduce the black luminance,i.e., luminance upon display of dark image, thereby improving the totalcontrast ratio of the LCD device. JP-1989-10223A describes a multi-panelLCD device that achieves a contrast ratio exceeding the contrast ratioof a LCD device having a single LCD panel, which fact is confirmed bymeasuring the overall contrast ratio of the LCD device by using laser.It is described therein that two-panel LCD device achieves improvementof contrast ratio up to about 100:1 by using LCD panels having acontrast ratio of about 10 to 15:1, and that three-panel LCD deviceachieves a contrast ratio of 1000:1.

The technique of the multi-panel LCD device is also described in PatentPublications JP-2004-512564A and JP-2001-201764A. The techniquedescribed in JP-2004-512564A does not relate to a LCD device realizing ahigher contrast ratio, and related to the technique of automaticstereoscopic image display. The technique described in JP-2001-201764Adoes not relate to a LCD device realizing a higher contrast ratio, andrelates to the technique of uniquely designed LCD device by using themulti-panel LCD device.

For driving a projection LCD device having the multi-panel LCDstructure, a common signal can be used to drive all the LCD panelswithout involving any problem, because these LCD panels pass the lightsubstantially perpendicular to the LCD panels to project an image on ascreen. However, such a common signal involves a problem in an ordinarydirect-view LCD device having the multi-panel LCD structure, wherein alight source emitting a scattering light is used for image display as inthe case of the LCD device using a backlight unit. The problem is thatthe distance or gap between adjacent LCD panels generates a parallaxdepending on the viewing angle of an observer, the parallax preventingthe light passed by a pixel of the rear-side (or light-receiving-side)LCD panel from passing through the corresponding pixel of the front-side(or light-emitting-side) LCD panel. If the observer observes the displayscreen in a slanted viewing direction, the parallax deviates thedirection of the pixel of the front-side LCD panel from thecorresponding pixel of the rear-side LCD panel, and thus an edge of theimage in particular, across which the brightness of image issignificantly changed in general, will be observed as double lines.Thus, the observer feels a sense of discomfort.

FIGS. 23A, 23B, and 23C schematically show the image of pixels of themulti-panel LCD structure located at different positions of the displayscreen, as observed from the front center of the display screen. Themulti-panel LCD structure includes two LCD panels in this case. FIG. 23Ashows the image of a pixel located on the observers' left of the displayscreen, FIG. 23B shows another pixel located at the center of thedisplay screen, and FIG. 23C shows the image of another pixel located onthe observers' right of the display screen.

In the case shown in FIG. 23B, the pixel of the front-side LCD panel andthe pixel of the rear-side LCD panel are observed to exactly overlapeach other, without causing any problem. However, as shown in FIGS. 23Aand 23C, the pixel of the front-side LCD panel and the pixel of therear-side LCD panel are observed not to overlap each other, causing theproblem of reduction in the luminance. In addition, the deviation of thepixels generates a bright area and a dark area which are arrangedperiodically, to generate an interference pattern such as a moiré. Thus,the image quality of the LCD device is degraded. The dark area isgenerally formed by interconnect line or black matrix (hereinafter,simply referred to as black matrix) of the LCD panel which blocks thelight.

SUMMARY OF THE INVENTION

In view of the above problem of the conventional multi-panel LCD device,it is an object of the present invention to provide a multi-panel LCDdevice, which is capable of suppressing degradation in the image qualitycaused by a light interference between pixels of plurality of layeredLCD panels.

The present invention provides, in a first aspect thereof, a liquidcrystal display (LCD) device including: first and second LCD panels eachincluding a pair of transparent substrates and a liquid crystal (LC)layer sandwiched therebetween, the first and second LCD panels beingstacked one on another so that each pixel of the first LCD paneloverlaps a corresponding pixel of the second LCD panel; a pair of firstpolarizing films sandwiching therebetween the stacked first and secondLCD panels; and a light diffusion film having a light diffusing functionand at least one second polarizing film, which are interposed betweenthe first LCD panel and the second LCD panel.

The present invention provides, in a second aspect thereof, a liquidcrystal display (LCD) device including: first and second LCD panels eachincluding a pair of transparent substrates and a liquid crystal (LC)layer sandwiched therebetween, the first and second LCD panels beingstacked one on another so that each pixel of the first LCD paneloverlaps a corresponding pixel of the second LCD panel, the pixel of thefirst and second LCD panels including a bend and a pair of stripesextending form the bend, the bend of the pixel of the first LCD panel isdeviated in angular position from a corresponding bend of the pixel ofthe second LCD panel by a specific rotational angle.

The present invention provides, in a third aspect thereof, a liquidcrystal display (LCD) device including: first and second LCD panels eachincluding a pair of transparent substrates and a liquid crystal (LC)layer sandwiched therebetween, the first and second LCD panels beingstacked one on another so that each pixel of the first LCD paneloverlaps a corresponding pixel of the second LCD panel, wherein the pairof transparent substrates include an active substrate on which activedevices for driving the LCD layer are formed and a counter substrate,and at least one of the counter substrates of the first and second LCDpanels is interposed between the active substrate of the first LCD paneland the active substrate of the second LCD panel.

The present invention provides, in a fourth aspect thereof, a liquidcrystal display (LCD) device including: a backlight source, a firstpolarizing film, a first liquid crystal display (LCD) panel, at leastone second polarizing film, a second LCD panel and a third polarizingfilm, which are arranged in this order from a rear side toward a frontside of the LCD device; and at least one light diffusion film disposedin front of the first LCD panel.

In accordance with the LCD device of the first aspect of the presentinvention, the light diffusion film interposed between the first LCDpanel and the second LCD panel diffuses the light passed by the firstLCD panel and thus gradates the distinction between the bright area andthe dark area caused by black matrix of the first LCD panel, therebyalleviating the periodicity of the arrangement of the bright area andthe dark area caused by the deviation of the pixel between the first LCDpanel and the second LCD panel as observed in a slanted viewingdirection. Thus, the moiré caused by light interference is reduced tosolve the problem in the multi-panel CD device for achieving a highercontrast ratio.

In accordance with the LCD device of the second aspect of the presentinvention, the first and second LCD panels are stacked one on another sothat bend in the pixel of the first LCD panel is deviated from the bendin the pixel of the second LCD panel by the specific rotational angle.The deviation of the bend means that the portion of the pixel of thefirst LCD panel extending parallel to the pixel of the second LCD panelalleviates the periodicity of the bright area and the dark area as inthe case of the first aspect of the present invention, therebyalleviating the moiré caused by the light interference.

In accordance with the LCD device of the third aspect of the presentinvention, the active substrate of the first LCD panel on which theactive devices are formed is not disposed adjacent to the activesubstrate of the second LCD panel, whereby the light reflected by theactive device on the active substrate of the second LCD device is notreflected by the active device on the active substrate of the first LCDdevice. This prevents rainbow color caused by the light interferencefrom appearing in the emitted light.

In accordance with the fourth aspect of the present invention, the lightdiffusion film provided in front of the first LCD panel diffuses thelight passed by the first LCD panel to gradate the difference betweendark luminance and the bright luminance caused by the black matrix ofthe first LCD panel, to thereby reduce the spatial frequency of theimage on the display screen and thus reduces the moire.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description,referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the configuration of a LCD deviceaccording to a first embodiment of the present invention;

FIG. 2 is a schematic view of a moiré;

FIG. 3 is a schematic view of an alleviated moiré;

FIG. 4 is a sectional view of part of the LCD device, showing the lighttransmission in the LCD device;

FIG. 5 is a top plan view of a single pixel of a typical LCD device.

FIG. 6 is a sectional view of part of the LCD device, showing the lighttransmission in the LCD device;

FIG. 7 is a sectional view of part of the LCD device, showing occurrenceof moiré;

FIG. 8 is a sectional view of part of the LCD device, showing occurrenceof moiré.

FIG. 9 is a table showing the grade and the degree of moiré that theobserver feels;

FIG. 10 is a perspective view showing measurement of the luminancedistribution for different polar angles;

FIG. 11 is a graph showing the luminance distribution plotted againstthe polar angle;

FIG. 12 is a graph showing the relationship between the half-value angleand the grade of moiré;

FIG. 13 is a table showing the relationship between the grade and thedegree of moiré;

FIG. 14 is a graph showing the relationship between the half-value angleand the moiré reduction;

FIG. 15 is a graph showing the relationship between the constant K andthe luminance reduction ratio;

FIG. 16 is a sectional view showing a LCD device according to a secondembodiment of the present invention;

FIG. 17 is a top plan view showing a single pixel of a LCD panel in thesecond embodiment;

FIGS. 18A, 18B and 18C are top plan views showing the image of pixels ofthe multi-panel LCD structure located at different positions of thedisplay screen, as observed from the front center of the display screen;

FIG. 19 is a table showing the results of the display test of the LCDdevices of the first and second embodiments;

FIG. 20 is a sectional view showing the configuration of a LCD deviceaccording to a second embodiment of the present invention;

FIG. 21 is a sectional view showing the configuration of amultiple-panel LCD device according to a third embodiment of the presentinvention; and

FIG. 22 is a sectional view showing the configuration of amultiple-panel LCD device according to a fourth embodiment of thepresent invention.

FIGS. 23A, 23B and 23C are top plan views showing the image of pixels ofthe multi-panel LCD structure located at different positions of thedisplay screen, as observed from the front center of the display screen;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, embodiments of the present invention will be described in detailwith reference to the accompanying drawings. FIG. 1 is a sectional viewshowing the configuration of a LCD device according to a firstembodiment of the present invention. The LCD device, generallydesignated by numeral 10, includes a first LCD panel 11 as a rear-sideLCD panel, a second LCD panel 12 as a front-side LCD panel, a backlightunit 13 disposed at the rear of the first LCD panel 11 and a lightdiffusion layer 14 interposed between the first LCD panel 11 and thesecond LCD panel 12.

The first and second LCD panels 11 and 12 each include a pair oftransparent substrates opposed to each other with a predetermineddistance therebetween, a liquid crystal layer sandwiched between thetransparent substrates, and a pair of polarizing films each arranged onthe surface of a corresponding one of the transparent substrates farfrom the liquid crystal (LC) layer. Further, at least one of the firstand second LCD panels 11, and 12, for example, the second LCD panel 12,includes color filters.

As the display mode of both the first and second LCD panels 11 and 12,lateral-electric-field-mode such as IPS mode may be exemplified. If anIPS-mode LCD panel is used, the pair of polarizing films have lighttransmission axes (polarizing axes) perpendicular to each other. Inaddition, the polarizing film of the first LCD panel on the lightemitting side or front side thereof has a polarizing axis (transmissionaxis) is disposed parallel to the transmission axis of the polarizingfilm of the second LCD panel on the light receiving side or rear sidethereof.

The backlight unit 13 is configured as a display light source for theLCD device 10. The first and second LCD panels 11 and 12 are stacked oneon another so that the positions of the corresponding pixels exactlyoverlap each other. Further, the LCD panel are stacked so that the lighttransmission axis or light absorption axis of the polarizing film on thelight emitting side or front side of the first LCD panel 11 issubstantially parallel to the light transmission axis or lightabsorption axis of the polarizing film on the light receiving side orthe rear side of the second LCD panel 12. For the first and second LCDpanels 11 and 12, the display of each pixel is controlled based on thesame image data. The light diffusion layer 14 is inserted between thefirst LCD panel 11 and the second LCD panel 12. The light diffusionlayer 14 diffuses the light emitted from the backlight unit 13 andpassed by the first LCD panel 11 to allow the light to enter the secondLCD panel 12.

In the present embodiment, as described above, the light diffusion layer14 is inserted between the first LCD panel 11 and the second LCD panel12. Diffusion of the light passed by the first LCD panel 11 by using thelight diffusion layer 14 gradates the distinction between the brightarea and the dark area, which are generated by the presence ofinterconnect lines or black matrix upon passing the first LCD panel 11.This alleviates the periodicity of the bright area and the dark area,generated by the parallax due to the deviation of the pixel of the firstLCD panel from the second LCD panel upon observing the display screen ina slanted viewing direction, alleviating the moiré occurring due tolight interference. Accordingly, while achieving a higher contrast ratioby using a multi-panel LCD device, deterioration of the image qualityupon observing the LCD device in the slanted direction is avoided.

If the first LCD panel and the second LCD panel are arranged so thatboth the active panels or TFT panels, on which active devices arearranged, are disposed adjacent to each other without insertion of thelight diffusion film, a rainbow color will be observed due to the lightinterference. The light diffusion film interposed between the first LCDpanel and the second LCD panel can alleviate the problem of the rainbowcolor by separating the TFT panels from each other.

In the multiple-panel LCD device of the present invention, each LCDpanel includes a pair of transparent substrates each including anorientation film and a LC layer sandwiched therebetween with theorientation film in contact with the LC layer. Each of the LCD panelsincludes a pair of polarizing films having optical axes intersectingwith each other at an angle of 90 degrees.

The first and second LCD panels may be IPS-mode LCD panel wherein LCmolecules in the LC layer are rotated in a plane parallel to thetransparent substrates sandwiching therebetween the LC layer to achievea bright state (light transmission) and a dark state (light interrupt).

The moiré is generated by a difference between two spatial frequencies.The problem of the moiré to be solved by the present invention is suchthat generated due to the difference between the spatial frequenciesformed by the black matrix in the first and second LCD panels, thespatial frequencies causing the light interference. FIG. 2 shows thepseudo spatial frequencies generated in the multiple-panel LCD device.The stripe 51 corresponds to the image generated by the black matrix inthe second LCD panel 12 shown in FIG. 1, whereas the stripe 52corresponds to the image generated by the black matrix in the first LCDpanel 11 shown in FIG. 1.

Comparing the stripe 52 against the stripe 51, the stripe 52 has asmaller width and a smaller pitch than the stripe 51. In the structureshown in FIG. 1, it is understood from the fact that the first LCD panel12 generating the stripe 52 is disposed farther than the second LCDpanel 11 generating the stripe 51 from the observer, with a gap beingdisposed between the first LCD panel and the second LCD panel, therebycausing an apparently smaller dimension of the first LCD panel due tothe perspective effect. Even if both the LCD panels 11 and 12 have anexactly same structure, the gap generates a difference in the spatialfrequency between the first LCD panel and the second LCD panel, thedifference being significantly larger than the actual difference in theoriginal spatial frequency and observed as moiré.

FIG. 3 shows modification of FIG. 2 by gradating or averaging the stripe52 shown in FIG. 2. Considering application of the above perspectiveeffect to the actual LCD device, the moiré shown in FIG. 2 is alleviatedin FIG. 3 and thus is less conceived by the observer. More specifically,FIG. 2 shows a larger difference in the luminance between the brightline and the dark line, thereby indicating a larger contrast ratio. Onthe other hand, FIG. 3 shows a smaller difference in the luminancebetween the bright line and the dark line of the stripe 53 due to thegradating or averaging of the stripe 53. The smaller difference in theluminance of the stripe 53 alleviates the light interference between thestripe 51 and the stripe 53, to alleviate the moiré. In other words, thegradating of the stripe 52 allows the boundary between the bright lineand the dark line to be vague and obfuscate the pattern of the stripe52, to alleviate the moiré. As described heretofore, the moiré can bealleviated by reducing the amplitude or contrast ratio of one of thespatial frequencies which cause therebetween the light interference ormoiré. For investigating the condition for and degree of alleviation inthe moiré, simulation was conducted using a specific model.

FIG. 4 shows the model of the multiple-panel LCD device used for thesimulation. The model of the LCD device includes a light diffusion film62, a polarizing film 63, a glass substrate 64, and a black matrix 65,and is irradiated by a light source 61. It is defined herein that d1(meters), θ (degree) and c1 are the distance between the black matrix 65and the light diffusion film 62, a diffusion angle of the light effectedby the light diffusion film 62 and the point at which the perpendicularto the panel from the light source 61 intersects the plane on which theblack matrix 65 is formed, respectively. In this definition, thediffusion angle θ of the light is the half-value angle which provides alight transmission half the maximum light transmission obtained at thepoint c1.

FIG. 5 shows an exemplified structure of the pixel as observed by theobserver, the pixel having a black matrix 65 by which the light isstopped and an opening 67 through which the light passes. It is assumedthat the point c1 is located on a line segment A-A′ or the line segmentB-B′ crossing the black matrix 65 and opening 67, and the case of thepoint c1 being located on the line B-B′ is discussed hereinafter.

In FIG. 4, it is assumed that the light source 61 emits light in thedirection perpendicular to the panel, and diffused by the lightdiffusion film 62 to advance toward the plane on which the black matrix65 is formed while being diffused within the sector region having acentral angle of 2θ. The thus diffused light forms a shape of circularcone having an apex of 2θ after reaching the plane on which the blackmatrix 65 is formed. In other words, the light is diffused within acircular region having a center at the point c1, at which theperpendicular from the light source 61 to the plane intersects theplane.

Considering the reversible nature of the light, as shown in FIG. 6, ifthe observer 66 is located at the light source 61 shown in FIG. 4, andwhen the observer looks at the point e1, the observer observes the lightpassed through the circular region having a center at c1 and a diameterof w1 meters. The diffusion performance of the light diffusion film islarger if the half-value angle θ effected by the light diffusion film islarger, and smaller if the half-value angle θ is smaller. In otherwords, a higher diffusion performance of the light diffusion filmenlarges the circular area having a diameter of w1, whereby the observerlooking at the point c1 observes the light passed through a larger area.

The diameter w1 is calculated by the following formula:w1=2×d1×tan θ  (1).From the formula (1), the half-value angle θ is obtained by thefollowing equation:θ=tan⁻¹(w/2d1)  (2).

Considering that the black matrix 65 has a width of a1 meters at eachstripe thereof and a the gap between adjacent stripes of the blackmatrix 65 is b1 meters, the way in which the observer observes the imagewill be discussed hereinafter while assuming different cases.

FIG. 7 shows the first case where w1<a1+b1 in the above situation. Ifthe observer 66 moves parallel to the LCD panel, the relationshipw1<a1+b1 allows the luminance of the circular region having a diameterof w1 to be changed between the minimum at which the circular regionoverlaps the black matrix 65 at a maximum and the maximum at which thecircular region overlaps the gap at a maximum. The change of theluminance depending on the viewing point means a larger amplitude of thespatial frequency, such as shown in FIG. 2, thereby causing the moiré.

FIG. 8 shows the second case where w1>a1+b1. The relationship w1>a1+b1restricts the change of the luminance, if the observer moves parallel tothe LCD panel, because the circular region having a diameter of w1overlaps the gap b1 of the black matrix 65 at any time. That is, theluminance of the second case changes in an amount significantly smallerthan the amount of change of the first case. If the relationship w1

a1+b1 holds in particular, the change of the luminance will benegligibly small. Thus, in the second case, the amplitude of the spatialfrequency is small, as shown in FIG. 3, whereby alleviation of the moiréis expected.

In view of the above, the degree in which the moiré occurring in the LCDpanel is observed was evaluated in a subjective five-grade evaluation bya plurality of persons, as shown in the table of FIG. 9. In thisevaluation, grade 5 means presence of no moiré, grade 4 means presenceof a small degree of moiré which the observer scarcely conceives, grade3 means presence of a significant degree of moiré which the observernotices and yet does not feel discomfort, grade 2 means presence ofconsiderable degree of moiré by which the observer fees some discomfort,and grade 1 means presence of a large degree of moiré which the observerfeels a large discomfort to observe the image on the display screen.

A LCD panel was prepared in which a1=77 μm, b1=193 μm and d1=900 μm, anda variety of light diffusion films having different half-value angleswere used. The LCD device was observed for the degree of moiré. Theluminance distribution along the polar angle was measured using LCD 7000(trade mark, from Otsuka Densi co.) for the LCD panel which wasevaluated at grade 3 in the state of using the light diffusion film.FIG. 16 shows the situation of the measurement.

In FIG. 10, a light diffusion film 72 sandwiched between a pair of glassplates 71 is disposed against a light source 73 so that the light source73 emits light perpendicular to the diffusion film 72, and an opticalsensor 74 is disposed in an opposing relationship with respect to thelight source 73, with an intervention of the light diffusion film 72therebetween. The light source 73 emitted a linearly-polarized light ata constant intensity, and the optical sensor 74 measured the intensityof the light transmitted through the light diffusion film 72 at a polarangle (denoted by numeral 75) between zero degree and 60 degrees. Thetransmittance of the light diffusion film 72 is determined at a polarangle of zero degree.

FIG. 11 shows the luminance distribution plotted with respect to thepolar angle of the optical sensor between −60 degrees and +60 degreesfor the case of a light diffusion film evaluated at grade 3. Asunderstood from FIG. 11, the luminance assumes a maximum at a polarangle of zero degree, and reduces along with increase of the polarangle, providing a Gaussian curve distribution. Numeral 82 denotes thehalf value which is half the maximum luminance at the polar angle ofzero degree, and is equal to the luminance at a polar angle of 4.2degrees, which was evaluated at grade 3 for the light diffusion film.Thus, the angle θ obtained by formula (2) is defined herein as thehalf-value angle, and is used as a factor for evaluating the diffusingperformance of the light diffusion film.

The relationship between the hall-value angle as described above and thedegree of moiré observed during using a variety of light diffusion filmswas then examined. FIG. 12 shows the result of examination, whereingrade number of moiré is plotted against the half-value angle in termsof degrees. As understood from FIG. 12, grade 1 corresponds to ahalf-value angle of 2.0 degrees, grade 2 corresponds to a half-valueangle of 2.4 degrees, and grade 3 at which the observer does not feeldiscomfort by the presence of moiré in the image corresponds to ahalf-value angle of 4.2 degrees. It is also understood that the grade 4,at which the observer conceive the presence itself of the moiré and yetdoes not feel discomfort during observing the image on the screen,corresponds to a half-value angle of 6.0 degrees or above, and ahalf-value angle of 8.2 degrees or above is evaluated at grade 5 atwhich the moiré itself is not conceived.

The limit of half-value angle at which the moiré is not conceived by theobserver was then examined using simulation and the degree of moiréobserved. In this examination, the degree of reduction in the moiré wasobtained based on the half-value angle of a variety of light diffusionfilm, providing the result shown in FIG. 13, which shows thecorrespondence between the grade number and the reduction in the moiré(moiré reduction). The moire reduction is defined herein by a ratio ofthe difference between the brightest luminance and the darkest luminancein the moiré to the average luminance. More specifically the moirereduction is defined by the following formula:MR=20×log₁₀(B/A),where B and A are the amplitude of moire and mean luminance,respectively. FIG. 13 shows the relationship between the half-valueangle and the moiré reduction, based on which the degree of moire in theobservation can be evaluated for the specific amplitude of moire withrespect to the mean luminance.

Thereafter, the relationship between the half-value angle of the lightdiffusion film and the moire reduction is obtained. FIG. 14 shows therelationship between the half-value angle and the more grade. In FIGS.13 and 14, the result of the simulation shows that the half-value anglecorresponding to a moiré reduction of −18 dB which corresponds to grade1 is 1.7 degrees, the half-value angle corresponding to a moiréreduction of −18 dB which corresponds to grade 1 is 1.7 degrees, thehalf-value angle corresponding to a moiré reduction of −19 dB whichcorresponds to grade 2 is 2.4 degrees, the half-value anglecorresponding to a moiré reduction of −21 dB which corresponds to grade3 is 4.2. degrees, the half-value angle corresponding to a moiréreduction of −23 dB which corresponds to grade 4 is 5.8 degrees, and thehalf-value angle corresponding to a moiré reduction of −26 degrees whichcorresponds to grade 5 is 7.9 degrees. Thus, the result of simulationsubstantially coincides with the result of measurements, revealing thecorrectness of the theoretical analysis of the present invention.

The LCD panel as described above, i.e., the LCD panel in which a1=77 μm,b1=193 μm, and d1=900 μm is used, with the relationship: w1=a1+b1=270 μmbeing additionally satisfied, for calculating the half-value angle θ forthe case of observing the circular region having a diameter w1 includingthe black matrix and the opening from the formula (2). The result ofcalculation provided θ=8.5 degrees.

Based on the result, θ=8.5 degrees as a reference, the constant K isdetermined from the half-value angle at which the grade number isdefined. More specifically, by using the relationship:θ=K×tan⁻¹{(a1+b1)/2d1)}  (3),the value of constant K for each grade number is obtained by the ratioof the half-value angle obtained from FIG. 12 to the referencehalf-value angle θ=8.5 degrees.

For example, grade 1 corresponds to a half-value angle θ=1.7 degrees inFIG. 12, whereby K=0.20 is obtained by the ratio of 1.7/8.5. In thisway, the following results:

K=0.20 for grade 1;

K=0.28 for grade 2;

K=0.50 for grade 3;

K=0.68 for grade 4; and

K=0.93 for grade 5

are obtained.

From the above results, assuming that K is a constant for each gradenumber, if a light diffusion film satisfying the relationship:θ≧K×tan⁻¹{(a1+b1)/2d1)}  (4)is used, a desired grade can be used. More concretely, by employing thevalue for K corresponding to grade 3, i.e., K=0.50 or above, a desiredstate in which the observer does not feel discomfort in the moire isobtained. Thus, a desired performance of the light diffusion film forsuppressing the moiré, the constant K should be 0.50 or above forachieving grade 3, preferably 0.68 or above for achieving grade 4, andmore preferably 0.93 or above for achieving grade 4.

In the view point of solving the moiré, there is no upper limit of K,and thus a larger K provides a more effective reduction in the moiréHowever, an extremely larger value for the constant K reduces the frontluminance, i.e., luminance in the direction normal to the displayscreen, because the front light intensity is excessively reduced by thelarge K or higher diffusion performance, although the light intensity ofthe light source is constant.

The reduction ratio (RR) of the luminance by inserting the lightdiffusion film is defined from the luminance (B1) before insertion ofthe light diffusion film and the luminance (B2) after insertion of thelight diffusion film as follows:RR={(B1−B2)/B1}×100(%)FIG. 15 shows the relationship between the reduction ratio of theluminance and the constant K. As understood from FIG. 15, the reductionratio is proportional to the constant K, and defined by:RR=14.4×K  (5)Accordingly, the upper limit for K is determined by the reduction ratioRR of 100%, which provides 6.9 for K in the graph. Thus, the upper limitof K is considered at 6.9.

Examples of the material for the light diffusion film in the presentinvention include a surface diffusing film such as the optical diffusingsheet described in Patent Publication JP-1994-64604A. The opticaldiffusing sheet described therein includes a base film and a pluralityof embedded films embedded within the base film. The embedded films aresuch that bead-like particles having different diameters around 1 to 500μm and nonuniform surface are mixed to have different grain sizes.Instead of the embedded films, a plurality of protruding films having asimilar configuration may be provided on the surface of the base film byattachment thereto with adhesion. The optical diffusion film has a totaltransmission factor of 85 to 88% and a haze controlled in a wide rangebetween 49% and 70%.

Kimoto corp. provides a “Light-up Series”, as the material for the lightdiffusion film, including 100PBA, 75PBU, 38NSH, 100NSH, 100SXE, 50MXE,100MXE, 38LSE, 50LSE, 75LSE, 100LSE, 188LSE, 100GM2, 188GM2, 100GM3,188GM3, 50UK2, 100UK2, 125TL2, 125TL4, 50UK4, 100UK4, 100DX2, and188DX2, which provide a total transmission factor between 74.0% and 97%,and a haze of 29.0% to 92.0% (catalogue value). Keiwa corp. provides an“Oparus Series” including BS-910, BS-911, BS-912, BS-913, BS-700,BS-701, BS-702, BS-04, BS-042, BS-510, BS-511, BS-512, PBS-620N,PBS-620W, PBS-620HG-N, PBS-620HG-W, PBS-070L, PBS-071L, PBS-072L,PBS-070, PBS-071, PBS-072, PBS-070H, PBS-071H, PBS-072H, ZD-007,PBS-067, BS-506, BS-046, BS-036, BS-017, and ZD-097, which provides aswide a haze as 12.8% to 89.7% (catalogue value).

Examples of the material for the light diffusion film include an opticaldiffusion adhesive sheet described in Patent Publication JP-2006-16515A.The optical diffusing adhesive sheet includes an adhesive layer having alight diffusing function. The optical diffusing adhesive layer includesacrylic copolymer having a refractive index of n1, inorganic particleshaving a refractive index of n2 and an average grin size of 1 to 5 μm,and a curing agent. The optical diffusing adhesive layer is obtained byadding 0.1 to 50 weight part of organic particles to 100 weight part ofacrylic copolymer and adding thereto 0.01 to 15 weight part of curingagent to prepare optical diffusing adhesive, and coating a plastic filmwith the optical diffusing adhesive on at least one side thereof. Thedifference |n1−n2| in the refractive index is set within 0.01 and 0.2 toachieve a haze of 50% or above and a total transmission factor of 80% orabove in the optical diffusing adhesive layer.

The light diffusion film 14 may be made of optical diffusing adhesiveagent described in Patent Publication JP-1999-508622. The opticaldiffusing adhesive layer includes a pressure-sensitive base materialhaving a refractive index of n1 and filled with organic polymerparticles having a refractive index of n2, with the difference |n1−n2|in the refractive index being 0.01 to 0.2, wherein the weight ratio ofthe base material to the organic polymer particles is 1:1 to 50:1. Theorganic polymer particles have a diameter of 0.5 to 30 μm, and thepressure-sensitive base material is formed as spherical particles havinga diameter of 0.5 to 150 μm. The pressure-sensitive adhesive agent has aspecific characteristic determined by the concentration of the organicpolymer particles, refractive index difference, the balance between thethickness and the grain size of the diffusion material.

Examples of the material for the light diffusion film 14 include otheradhesive sheet, binding agent and organic synthetic resin so long asthey have a light diffusing function. The light diffusing film preparedby those materials may be provided at any position in front of, i.e., onthe light emitting side of the first LCD panel, to provide the moiréreduction function.

The light diffusion layer 13 preferably has the function of maintainingthe original polarized state of the incident light and diffusing theincident light. The reason is as follows. If a linearly-polarized lightpassed by the first LCD panel 11 has a polarized state after passingthrough the light diffusion layer 12, the thus polarized light may havea light component blocked by the light-incident-side polarization filmof the second LCD panel 12, thereby causing a loss of light afterpassing through the second LCD panel 12.

As a sheet having the functions of diffusing a linearly-polarized lightadvancing in a particular direction and maintaining the originalpolarized state of the linearly-polarized light, there is known amultiple reflection/diffusion sheet, which is formed by layering aplurality of films having different refractive indexes while allowingeach of the films to reflect some of the light. Examples of the lightdiffusion film 14 include DBEF (trade name) manufactured by 3M. In thecase using the DBEF as the light diffusion layer 14, the DBEF isarranged so that the light transmission axes of the DBEF, thelight-emitting-side polarizing film of the first LCD panel 11 and thelight-receiving side polarizing film of the second LCD panel 12 shouldbe directed parallel to one another. The DBEF used as the lightdiffusion layer 14 can reduce the loss of light and suppress reductionin the luminance, differently from a ordinary light diffusion filmwithout having the function of maintaining the polarized state of thelinearly polarized light.

In the present embodiment, on one hand, a higher light diffusionfunction of the light diffusion film 14 can alleviate the distinctionbetween the bright area and the dark area in a larger degree withrespect to the light passed by the fist LCD panel 11, therebyalleviating the moiré in a larger degree. On the other hand, however,the higher diffusion function reduces the luminance upon display of abright state on the screen due to a reduction in the light transmittanceof the light diffusion film 14. More specifically, there is a tradeoffbetween the alleviation of the moiré and the luminance upon display of abright state. For designing an actual LCD device, the light diffusionfunction of the light diffusion layer 14 should be determined inconsideration of a suitable balance between the luminance upon displayof a bright state and the degree of alleviation of the moiré.

JP-2004-512564A as described before recites that a light-reflectionpolarizing film which reflects the light component oscillating in anundesirable direction may be used instead of a light-absorptionpolarizing film to improve the luminance. JP-2001-201764A as describedbefore recites use of a DBEF film as a light-reflection polarizing film,which is disposed between two LCD panels stacked one on another.However, the DBEF film has a lower polarizing function as compared to ausual light-absorption polarizing film. Thus, the DBEF film replacingthe polarizing film between the two LCD panels cannot be expected toachieve a drastic improvement of the contrast ratio in themultiple-panel LCD device. In contrast thereto, in the presentembodiment, a DBEF film configuring the light diffusion layer 14 isprovided, in addition to the polarizing film, between the firs LCD panel11 and the second LCD panel 12. This configuration achieves a highercontrast ratio, and also suppresses degradation in the image qualitycaused by the light interference between the two LCD in a slantedviewing direction.

FIG. 16 is a sectional view of a LCD device 10 a according to a secondembodiment of the present invention. The LCD device 10 a of the presentembodiment is configured such that the light diffusion layer 14 isremoved from the LCD device 10 of the first embodiment shown in FIG. 1.In the present embodiment, the pixels of the first and second LCD panels11 and 12 having a specific structure remove the moiré caused by theinterference as observed in a slanted viewing direction. The specificstructure of the pixels will be described hereinafter. It is assumedherein that the first and second LCD panels 11 and 12 are IPS-mode LCDpanels.

FIG. 17 is a top plan view of a typical pixel in the LCD panels 11 and12 in the second embodiment. The pixel shown in FIG. 3 belongs to thefirst LCD panel 11, for example. The pixel is associated with a signalline or gate line 21 extending along a row direction 301, a data line 22extending along a direction 302, and a TFT (thin-film transistor) 24disposed in the vicinity of an intersection between the signal line 21and the data line 22. On/off of the TFT 24 is controlled by thepotential of the signal line 21.

The pixel includes a pixel electrode having comb-shape teeth 25 andconnected to the data line 22 via the TFT 24, and a common electrodehaving comb-shape teeth 26 and surface electrode portion 27 which areconnected to an inter-pixel common electrode line 23. Within the pixelarea, the comb-shape teeth 25 of the pixel electrode oppose thecomb-shape teeth 26 and surface electrode portion 27 of the commonelectrode to drive the LC layer by means of an electric field generatedby the potential difference between the pixel electrode and the commonelectrode.

The comb-shape teeth 25 of the pixel electrode, and the comb-shape teeth26 and surface electrode portion 27 of the common electrode extend inthe column direction 302 and have a single bend at the central positionthereof. More specifically, the comb-shape teeth 25 of the pixelelectrode, as well as the comb-shape teeth 26 and surface electrodeportion 27 of the common electrode, are an angle of −15° (or 165°)tilted away from the column direction 302 toward the row direction 301in the upper area of the pixel, as viewed from the top side toward thebottom side of the pixel, and are an angle of +15° tilted away from thecolumn direction 302 toward the row direction 301 in the lower area ofthe pixel, whereby the extending direction of these electrode portionsare an angle of +30 degrees bent within the pixel area as viewed fromthe top side toward the bottom side. It is to be noted that the sign ofthe angle is expressed in plus as viewed in the right along the rowdirection. Although a single bend is formed in each electrode portion inFIG. 3, a plurality of bends may be provided in each electrode portion.

In the second LCD panel 12, the comb-shape teeth of the pixel electrode,as well as the comb-shape teeth and surface electrode portion of thecommon electrode, are bent within the pixel. However the extendingdirection and the bend of the electrode portions are 90 degrees awayfrom the extending direction and the bend of the first LCD panel 11. Inother word, the electrode portions of the second LCD panel 12 has astructure which is obtained by rotating the structure of FIG. 3 by anangle of 90 degrees.

FIGS. 18A, 18B and 18C show the structure of the electrode portions ofthe first and second LCD panels in the multi-panel LCD device of thepresent embodiment. More specifically, FIG. 18A shows the structure ofelectrode portions of the pixel located on the observers' left of thedisplay screen, FIG. 18B shows the structure of the electrode portionsof the pixel located at the center of the display screen, and FIG. 18Cshows the structure of the electrode portions of the pixel located onthe observers' right of the display screen. In these figures, the solidline 41 denotes the pixel and the electrode portions of the first LCDpanel 11, whereas the dotted line 42 denotes the pixel or electrodeportions of the second LCD panel 12. It is to be noted that each solidline 41 or each dotted line 42 depicted within the contour solid line ordotted lines represents one of the comb-shape teeth 25 of the pixelelectrode, and the comb-shape teeth 26 and surface electrode portion 27of the common electrode. In this structure, the electrode portions ofthe first LCD panel are obtained by rotating the corresponding electrodeportions of the second LCD panel by an angle of 90 degrees.

Upon observing the LCD device 10 a in the direction normal to the LCDpanels 11 and 12, as shown in FIG. 18B, the pixel 41 of the first LCDpanel 11 and the pixel 42 of the second LCD panel 12 are superposedsubstantially in the same area. If the viewing angle is switched and theLCD device 10 a is observed in a slanted direction, the distance betweenthe observer and the first LCD panel 11 is different from the distancebetween the observer and the second LCD panel 12. Therefore, uponobserving the pixel located on the observers' left by the observerlocated on the front center of the display screen, the area of the pixel41 is deviated from the area of pixel 42, as shown in FIG. 18A.Similarly, upon observing the pixel located on the observers' right bythe observer located at the front center of the display screen, the areaof the pixel 41 is also deviated from the area of pixel 42, as shown inFIG. 18C.

Comparing FIG. 18B showing the pixel observed from the front againstFIGS. 18A and 18C showing the pixel observed in the slanted direction,there is no substantially difference in the state of overlapping of thecomb-shape teeth 25 of the pixel electrode, comb-shape teeth 26 andsurface electrode portion 27 of the common electrode of the pixels 41and 42 between the front view and the slanted view. Therefore, thedifference in the luminance between the front view and the slanted viewis reduced in the present embodiment as compared to the case ofoverlapping shown in the conventional structure of FIG. 23, wherein tworectangular pixels are overlapped. This means the luminance has asubstantially no difference between the front view and the slanted view.

Further, for the pixel 41 of the first LCD panel 11 and the pixel 42 ofthe second LCD panel 12, the bending directions of the pixels are 90degrees deviated from each other, thereby eliminating the portion of thecomb-shape teeth 25 of the pixel electrode, comb-shape teeth 26 andsurface electrode portion 27 of the common electrode of the pixel 41extending parallel to the electrodes of the pixel 42. In this manner,the periodicity of the arrangement of the bright area and the dark areais alleviated, thereby removing the problem of the interference fringessuch as the moiré.

FIG. 19 is a table showing the results of the display test of the sampleLCD devices of the first and second embodiments, the test resultincluding the luminance upon display of bright image and presence orabsence of moiré. In this figure, the result of the display test ofcomparative examples is also illustrated. The first sample included thelight diffusion film in the LCD device according to the firstembodiment, the second sample included the DBEF film in the LCD deviceaccording to the first embodiment, and the third sample had thestructure of shown in FIGS. 18A, 18B and 18C according to the secondembodiment. The first comparative example was such that the second LCDpanel had a pixel obtained by mirror reversing the pixel shown in FIG.17, with the pixel of the first LCD panel having the structure of FIG.17. The second comparative example was such that the first LCD panel hada pixel obtained by mirror reversing the pixel shown in FIG. 17, withthe pixel of the second LCD panel having the structure of FIG. 17.

Focusing on the luminance upon display of brightest image in the testresult shown in FIG. 19 reveals that the second sample including theDBEF film and the third sample, i.e., the second embodiment as well asthe first and second comparative examples provided a superior result orrelatively superior result, with the first sample being inferior to someextent to the second and third samples and the first and secondcomparative examples. In the result of the luminance upon display ofbrightest image, it is noted that the DBEF film which diffuses theincident light while maintaining the polarized state thereof cansuppress reduction in the luminance upon display of the brightest imagein a larger degree than the light diffusion film which simply diffusesthe incident light.

On the other hand, focusing on the moiré shown in FIG. 19 reveals thatthe first through third samples of the embodiments provided a superiorresult, compared to the first and second comparative examples whichgenerated moiré. This test result reveals that the comparative examples,wherein the pixel structure of the first LCD panel corresponds to thepixel structure obtained by mirror reversing the pixel structure of thesecond LCD panel, do not effectively reduces the periodicity of thearrangement of the bright area and the dark area because the mirrorreverse does not sufficiently remove the portion of the first LCD panelextending parallel to the portion of the second LCD panel. Thus, thefirst and second comparative examples had a poor image quality.

It is to be noted that although the multiple-panel LCD devices of theabove embodiments included IPS-mode LCD panels, the LCD panels of themultiple-panel LCD device are not limited to IPS-mode LCD panels. TheLCD panels may be TN-mode or VA-mode LCD panels. If the LCD panels ofthese modes include a light diffusion film or DBEF film according to thefirst embodiment or include the pixel structure shown in FIG. 18A etc.,the difference in the luminance between the front view and the slantedview is suppressed and the generation of the moiré can be prevented.

In the second embodiment shown in FIG. 18A etc, both the pixels 41 and42 have an angle difference of 90 degrees therebetween. However, it isenough to remove the parallel component existing between the pixel ofthe first LCD panel and the pixel of the second LCD panel. Thus, theangle difference is not limited to 90 degrees, and any angle between 0degree and 180 degrees may be employed, although an angle around 90degrees is preferred.

Further, an example in which each of the first and second LCD panels 11and 12 is provided with a pair of polarizing films sandwichingtherebetween a pair of transparent substrates has been described in theabove embodiments. However, either one of the pair of polarizing filmprovided in the first LCD panel 11 near the second LCD panel 12 or oneof the pair of polarizing films provided in the second LCD panel 12 nearthe first LCD panel 11 may be omitted. For example, in FIG. 1, thepolarizing film of the first LCD panel 11 near the second LCD panel 12may be omitted. In this case, the light emitted from the fronttransparent substrate of the first LCD panel 11 is diffused by the lightdiffusion layer 14 and incident onto the liquid crystal layer of thesecond LCD panel 12 via the rear polarizing film and the reartransparent substrate of the second LCD panel 12. In this manner, evenif one of the polarizing films as described above is omitted, a similaradvantage can be obtained.

FIG. 20 shows the configuration of a multi-panel LCD device according toa third embodiment of the present invention. The LCD device includes abacklight unit 13, a first LCD panel 11 and a second LCD panel 12, whicharea arranged along the transmission direction of light as in the caseof the first and second embodiments. The first LCD panel 11 includes aTFT substrate 11 a, a LC layer 11 c and a counter substrate 11 b, whichare arranged in this order along the transmission direction of light.The second LCD panel 12 includes a counter substrate 12 a, a LC layer 12c and a TFT substrate 12 b, which are arranged in this order along thetransmission direction of light.

Generally, the TFT substrate of the LCD panel on which active elementssuch as TFTs are formed may be a light-receiving-side substrate or alight-emission-side substrate. However, if the TFT substrate of thefirst LCD panel 11 is arranged adjacent to the TFT substrate of thesecond LCD panel 12 differently from the present embodiment, the lightreflected by the TFTs on the TFT substrate of the second LCD panel 12 isreflected again by TFTs on the TFT substrate of the first LCD panel 11.These iterative reflections may generate a light interference, to createt rainbow color on the display screen.

In the present embodiment, both the TFT substrates 11 a and 12 b onwhich TFTs are formed are not arranged adjacent to each other In thisconfiguration, part of the light reflected by a TFT on the TFT substrate12 b of the second LCD panel 12 is absorbed by a black matrix formed onthe counter substrate 11 b or 12 a, and does not reach the TFT substrate11 a of the first LCD panel 11, thereby preventing occurring of rainbowcolor. This configuration is advantageous particularly in the secondembodiment in which the light diffusion layer is not provided betweenthe stacked LCD panels.

It is to be noted that the front luminance of the LCD device may bereduced by inserting the light diffusion film due to a lowertransmission factor in the front direction depending on the lightdiffusing performance of the light diffusion film 14. In view of thisproblem, a light condensing film may be provided in addition to thelight diffusing film 14 in the LCD device, for directing the light oncediffused by the light diffusing film 14 in the direction other than thefront direction again toward the front direction, or for condensing thelight in advance before the light diffusing film 14 diffuses the light.

FIG. 21 shows a multiple-panel LCD device according to a fourthembodiment of the present invention. The LCD device, generallydesignated by reference mark 10 c, has a two-panel structure similarlyto the first embodiment. The light diffusion film 14 and the lightcondensing film 15 are interposed between the first LCD panel 11 and thesecond LCD panel 12, with the light condensing film 15 being disposed infront of the light diffusion film 14. The light diffusion film 14 hasthe light diffusing function, as described heretofore, for diffusing thelight emitted by the backlight unit 13 and passed by the first LCD panel11 to pass the diffused light to the second LCD panel 12.

The light condensing film 15 condenses the diffused light, in which thedifference generated by the black matrix between the bright luminanceand the dark luminance is gradated, to direct the diffused light towardthe front direction while maintaining the gradated state thereof,thereby increasing the intensity of the front light once reduced by thelight diffusing function of the light diffusion film.

FIG. 22 shows a multiple-panel LCD device according to a fourthembodiment of the present invention. The LCD device, generallydesignated by reference mark 10 d, includes a backlight unit 13, a firstLCD panel 11, a light condensing film 15, a light diffusion film 14 anda second LCD panel 12. The light condensing film 15 condenses the lightemitted by the backlight unit 13 and passed by the first LCD panel, topass the light toward the light diffusing film 14. The light diffusionfilm 14 diffuses the condensed light to the second LCD panel, therebygradating the difference between the bright luminance and the darkluminance generated by the black matrix, while maintaining the condensedstate of the light having a higher transmittance.

Examples of the material for the light condensing film 15 include anoptical film such as described in Patent Publication JP-1999-508622. Theoptical film described therein mounts thereon a structure including aplurality of linear prisms. The linear prisms have a contained angle of70 to 110 degrees, and a contained angle of 90 degrees provides ahighest effect for the light condensing film 15. It is recited in thepublication that a pitch of the linear prisms is 10 to 100 μm, and apitch of 50 μm is efficient. The difference in the refractive indexgenerated at the interface between the linear prisms and the ambient aircondenses the light passed by the linear prisms toward the frontdirection. Sumitomo 3M corp. provides such a lens sheet, “BEF series”(trade mark).

The light condensing film 15 may be a multiple-reflection sheet such asa DBEF sheet, DBEF-II, provided from 3M corp. The multiple-reflectionsheet is such that a plurality of films having different refractiveindexes are layered one on another in a thickness direction, each of thelayered films having a specific light reflecting function. If DBEF-II isused as the light condensing film, the DBEF-II is disposed so that thelight transmission axis of the DBEF-II is parallel to the lighttransmission axis of the polarizing films provided on the front side ofthe first LCD panel 11 and on the rear side of the second LCD panel 12.

The multiple-reflection sheet and the light diffusion film may becombined to provide a single multiple-reflection diffusion film for usein the present embodiment. The multiple-reflection diffusion film hasboth the functions of the multiple-reflection sheet and the lightdiffusion film to achieve the moiré reduction and suppression of theluminance, and is provided from 3M corp. as “DBEF-D series”.

The light condensing film 15, if inserted on the front side of the lightdiffusion film, condenses the light which is diffused by the lightdiffusion film 14 to have gradated moiré, thereby achieving a higherluminance. The light condensing film 15, if inserted on the rear side ofthe light diffusion film 14, condense the light having moiré to increasethe luminance before diffusion by the light diffusion film 14, whichthen reduces the moiré.

The light diffusion film 14 and light condensing film 15 may be providedat any position so long as they are provided in front of the first LCDpanel 11, in order for reducing the moiré and increasing the front lightintensity. The order of the light diffusion film 14 and light condensingfilm 15 may be selected as desired. It is also possible to provide thelight diffusion film 14 and light condensing film 15 front side of thesecond LCD panel 12, with the order of these films 14 and 15 beingselected as desired. However, it is preferable that these films beinterposed between the first LCD panel 11 and the second LCD panel 12.

It is preferable that the pixel size of the first LCD panel be equal tothe pixel size of the second LCD panel. However, the first LCD panel mayhave a lower resolution than the second LCD panel, for example, may havea resolution half the resolution of the second LCD pane, for achievingthe advantages of the present embodiment including the moiré reductionand the luminance improvement.

With respect to the configuration wherein the light diffusion film 14and light condensing film 14 are provided for the moiré reduction andthe luminance improvement, a color filter is not an indispensableconstituent element in the present invention. That is, the LCD device ofthe present embodiment may be a monochrome LCD device. If a color LCDdevice is provided according to the present invention, color filterstherein are not limited to RGB color filters, and may include multipleRGBYMC color filters. In addition, a single pixel may be divided intofour regions, for example, which correspond to R, G, G, B colors. In analternative, the four regions may correspond to R, G, B, and achroma.

By stacking the second LCD panel onto the first LCD panel, for example,a gap is inevitably formed between the plane (film) on which the blackmatrix of the first LCD panel is formed and the plane (film) on whichthe black matrix of the second LCD panel is formed. This gap generates aparallax, which may be used for displaying a three-dimensional image onthe LCD device.

In the LCD panel of the above embodiment, electrodes are formed in amatrix on the surface of one of the transparent substrates near the LClayer, and each intersection between the electrodes of the matrix isprovided with a three-terminal element such as TFT, thereby configuringa single pixel. In each pixel, the pixel electrode connected to thethree-terminal element and the common electrode provided common to thearray of pixels are configured as comb-teeth electrodes for achieving alateral-electric-field LCD device such as an IPS-mode LCD device.However, the LCD device of the present invention is not limited to suchtype of the LCD device, and may have thin-film diodes (TFDs) instead ofTFTs. The LCD device may be driven by a simple-matrix driving scheme.

The LCD panel in the present invention may be any ofvertical-alignment-mode, twisted-nematic-mode, and bent-oriented-modeLCD LCD panels. A retardation compensation film may be provided betweenthe CLD panel and the light diffusion film to improve the viewing angledependency of the present invention.

The present invention may be applied to any type of the LCD deviceshaving a variety of modes or any image display system such as monitor TVin a broadcasting station, movie display system for use in a theater,and a monitor for a computer system.

Since the above embodiments are described only for examples, the presentinvention is not limited to the above embodiments and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope a the presentinvention.

1. A liquid crystal display (LCD) device comprising: first and secondLCD panels each including a pair of transparent substrates and a liquidcrystal (LC) layer sandwiched therebetween, said first and second LCDpanels being stacked one on another so that each pixel of said first LCDpanel overlaps a corresponding pixel of said second LCD panel; a pair offirst polarizing films sandwiching therebetween said stacked first andsecond LCD panels; and a light diffusion film having a light diffusingfunction and at least one second polarizing film, which are interposedbetween said first LCD panel and said second LCD panel, wherein a colorfilter is mounted on only one of the LCD panels among the first LCDpanel and the second LCD panel.
 2. The LCD device according to claim 1,wherein said light diffusion film is a multi-reflection light diffusionfilm including a plurality of layers each having a specific lightreflection function.
 3. The LCD device according to claim 1, whereinsaid light diffusion film diffuses a linearly-polarized incident lightwhile maintaining a linearly-polarized state of said incident light. 4.A two-dimensional liquid crystal display (LCD) device comprising: abacklight source, a first polarizing film, a first liquid crystaldisplay (LCD) panel, at least one second polarizing film, a second LCDpanel and a third polarizing film, which are arranged in this order froma rear side toward a front side of said LCD device, wherein at least onelight diffusion film is disposed in front of said first LCD panel, and acolor filter is formed on only one of the LCD panels among the first LCDpanel and the second LCD panel.
 5. The LCD device according to claim 4,wherein said first polarizing film has a transmission axis, which isparallel to a transmission axis of said third polarizing film andperpendicular to a transmission axis of said second polarizing film. 6.The LCD device according to claim 4, wherein the following relationshipholds:θ≧K×tan⁻¹{(a1+b1)/2d1} where d1, a1, b1, θand K are a distance betweensaid light diffusion film and a plane on which a black matrix nearest tosaid light diffusion film is formed, a width of stripes of said blackmatrix, a gap between adjacent stripes of said black matrix, ahalf-value angle representing a diffusion performance of said lightdiffusion film, and a constant equal to or above 0.50.
 7. The LCD deviceaccording to claim 6, wherein said K is equal to or above 0.68.
 8. TheLCD device according to claim 6, wherein said K is equal to or above0.93.
 9. The LCD device according to claim 4, further comprising atleast one light condensing film in front of said backlight unit.
 10. TheLCD device according to claim 9, wherein said light condensing film isdisposed in front of said first LCD panel.
 11. The LCD device accordingto claim 9, wherein said light condensing film includes a base film anda plurality of linear prisms arranged parallel to one another on saidbase film.
 12. The LCD device according to claim 1, wherein said firstand second LCD panels have a substantially equal resolution.
 13. The LCDdevice according to claim 1, wherein each of said pixels of said firstand second LCD panels is associated with a three-terminal active device,and said first and second LCD panels are driven by an active-matrixdriving scheme capable of driving using a pseudo static driving scheme.14. An image diagnosis device comprising the LCD device according toclaim 1.